1. Field of the Invention
This invention relates to a method for compressing and extending image signals wherein digital image signals are compressed when they are to be stored on a storage medium, such as an optical disk, or when they are to be transmitted between image signal processing apparatuses, and wherein the compressed image signals are then extended.
2. Description of the Prior Art
In general, digital image signals are composed of enormous amounts of information. Therefore, in general, before the digital image signals are stored on a storage medium or are transmitted between image signal processing apparatuses, they are compressed. As disclosed in, for example, U.S. Pat. Nos. 4,797,944, 4,805,030 and 4,807,042, and Japanese Unexamined Patent Publication Nos. 59(1984)-22845, 62(1987)-172883, 62(1987)-172884, 62(1987)-172885, 62(1987)-172886 and 62(1987)-247626, a prediction encoding process, an encoding process by orthogonal transformation, a vector quantization process, and the like, have been proposed as the processes for compressing and extending digital image signals. In the disclosed processes, redundancy of the image signals is restricted by the utilization of the characteristics that the image signal components representing the neighboring picture elements of a single image have a high correlation to each other.
When the image signals are stored on a storage medium or are transmitted between image signal processing apparatuses, the signal compressibility should preferably be as high as possible so that the required storage capacity can be reduced and the signal transmission rate can be kept high. However, in order for a high compressibility to be achieved, it is inevitable that an irreversible compression process be employed, with which the original image signal and a reconstructed image signal obtained from the compression of the original image signal and the extension of the compressed image signal are not identical with each other. In such cases, the image quality of an image reproduced from the reconstructed image signal becomes lower as the compressibility is increased. Therefore, the compressibility must be restricted such that the image quality of the reproduced image does not much deteriorate.
During the recording of images, operations for recording a set of images of a single object are often carried out. One type of the recording of a set of images is the angiography, with which several images of a single object injected with contrast media are recorded at short time intervals. The angiography is disclosed in, for example, U.S. Pat. No. 4,590,517. Another type of the recording of a set of images is the recording of contrasted images of the urinary channel of the renal pelvis, with which the contrasted images are recorded at long time intervals ranging from 10 to 20 minutes. A further type of the recording of a set of images is the simultaneous multilayer tomography, as disclosed in, for example, U.S. Pat. No. 4,581,535, with which several tomographic images of a single object are recorded simultaneously. A still further type of the recording of a set of images is the recording of images to be subjected to energy subtraction processing, as disclosed in, for example, U.S. Pat. No. 4,590,517.